Apparatus for extracting a glass ribbon from the egress of a flotation furnace

ABSTRACT

There is disclosed a flotation furnace for manufacturing glass ribbon and an apparatus for extracting the glass ribbon from the furnace. The apparatus includes a case with extraction rollers positioned therein. The case has an opening for receiving the glass ribbon from the furnace, an opening for exiting the glass ribbon from the case, and an opening for discharging combustion gases. The case may include a carriage which moves transverse to the flow of the glass ribbon.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the extraction of glass ribbon from aflotation furnace.

2. Description Of The Prior Art

The manufacture of glass ribbon by casting glass on a molten metal bathcontained in a flotation furnace is well known. In the known process,molten glass is poured onto a bath of molten metal to form a flat ribbonof glass. The metal bath is shielded by a protective reducing atmospherecomposed of hydrogen and nitrogen. The protective atmosphere is retainedwithin a leak-proof enclosure at slightly greater than atmosphericpressure. The glass ribbon is drawn from the flotation furnace byspecial extraction rollers whose height may be accurately positionedwith respect to the level of the metal bath. These rollers also conveythe glass ribbon to a flattening fixture for further treatment.

To ensure that the metal bath is protected by the reducing atmosphere,losses of the protective atmosphere as well as inflows of ambient air tothe flotation furnace must be minimized. These losses and unwantedinflows are acute at either end of the metal bath. To maximize retentionof the protective reducing atmosphere, the prior art devices have theirextraction rollers located in a lock-chamber of appreciable length whichcontains the same protective atmosphere as above the metal bath. Theupper portion of the lock-chamber is barred by a succession of leakproofcurtains arranged transversely above the extraction rollers. The lowerportion of the lock-chamber is barred by leak-proof joints positionedunderneath the lower generatrix of the rollers. The curtains and jointsform a leak-proof barrier which prevents the ambient atmosphere fromcontaminating the protective atmosphere. Thus, the leak-proof barrierprotects the metal bath from oxidation. Lock-chambers of this type haveseveral drawbacks. Access to various members such as the extractionrollers is difficult, making maintenance and replacement of the rollersarduous. Furthermore, the appreciable length of these lock-chambersresults in excessive heat loss from the glass ribbon as it passesthrough the lock-chamber. Consequently, the temperature of the glassribbon exiting the lock-chamber is often too low to permit furtherprocessing of the glass such as treating it with metal oxide deposits.

Other prior art devices have the extraction rollers positioneddownstream of the lock-chamber and exposed to the ambivient air. Accessto the rollers is facilitated by this arrangement but a substantialtemperature gradient between the furnace exit and the flattening fixtureentrance occurs. This gradient may alter the quality of the manufacturedglass ribbon.

We have invented an apparatus for extracting glass ribbon from theegress of a flotation furnace which provides easy access to theextraction rollers, retains the glass ribbon exiting the furnace at atemperature sufficiently high to permit further processing and maintainsa uniform thermal transition of the glass ribbon passing from thefurnace exit to the flattening fixture entrance.

SUMMARY OF THE INVENTION

This invention provides an improved apparatus for extracting a glassribbon from the egress of a flotation furnace. The apparatus comprises aplurality of extraction rollers arranged in succession adjacent theegress of the flotation furnace for extracting the glass ribbon from thefurnace and transporting it away from the furnace. The extractingapparatus further comprises enclosure means arranged adjacent the egressof the flotation furnace for enclosing the rollers. The enclosure meanshas an opening for receiving the glass ribbon, an opening for exitingthe glass ribbon, and an opening located above the rollers and adjacentthe egress of the flotation furnace which allows the combustion gases toexhaust to the ambient atmosphere. The enclosure means at leastpartially regulates the temperature of the glass ribbon passingtherethrough.

In an embodiment of the present invention, the enclosure means comprisesa case having a U-shaped cradle and an inverted U-shaped roof positionedabove the cradle. The U-shaped cradle includes side walls which areparallel to the axis of the glass ribbon. The upper edges of the sidewalls terminate at a level at least equal to that of the uppergeneratrix of the extraction rollers. The extraction rollers arerotatably mounted in succession within the cradle with their rotationalaxes transverse to the direction of the glass ribbon flow. The cradle atleast partially supports the rollers.

The U-shaped roof has side walls which are parallel to the axis of theglass ribbon. The side walls of the roof rest upon the side walls of thecradle. Advantageously, the end of the roof nearer the flotation furnaceis positioned downstream from the extraction roller nearest theflotation furnace so as to define the combustion gas vent openinggenerally between the egress of the flotation furnace and this end ofthe roof. Alternatively, the case is of a unitary construction with itsupper portion near the flotation furnace removed to define thecombustion gas vent opening.

The upstream and downstream ends of the roof may be sealed by sealingmeans such as gas-proof curtains. These curtains assist in preventing anabrupt thermal transition and in maintaining a uniform thermal gradientfor the glass ribbon passing through the case.

In one embodiment, the cradle has at least one of its upstream anddownstream ends closed by a transverse plate or wall. The upper edge ofthe wall (or walls) terminates at a level below that of the uppergeneratrix of the extraction rollers to permit the glass ribbon to passthereabove.

The cradle may be a carriage having means to move the carriagetransversely to the flow of the glass ribbon. For example, rollers maybe secured to the underneath portion of the carriage and movably mountedon guide rails which are positioned transverse to the direction of theglass ribbon flow. The mobility of the cradle provides easy access tothe extraction rollers for their maintenance and the like.

The extraction rollers may be located and positioned within the cradleof the casing so as to form a gap at the egress of the flotation furnacebelow the glass ribbon. Gases pass from the flotation furnace, throughthe gap and ignite. The resulting combustion gases pass at leastunderneath the extraction roller adjacent the flotation furnace,surround the glass ribbon and exit through the combustion gas ventopening in the case. A thermal barrier is thereby created between theflotation furnace and the case which aids in the thermal regulation ofthe glass ribbon. This produces an environment that heats the upper andlower surfaces of the glass ribbon passing through the case to auniformly high temperature.

In a preferred embodiment, the apparatus of the present inventionincludes a means for regulating the temperature of the egress of theflotation furnace comprising a cooling grid secured to the flotationfurnace egress below the glass ribbon. The cooling grid may include aplurality of pipes such as cold pipes independently supplied with acooling fluid for cooling the flotation furnace egress below the glassribbon. As will be explained below, it is desirable to cool this portionof the flotation furnace to protect it from attack by tin which iscontained in the molten metal bath of the flotation furnace.

Since it is undesirable to abnormally cool the extraction rolleradjacent the flotation furnace, a thermal screen may be positionedbetween the cooling grid and this extraction roller.

The opening in the egress of the flotation furnace may be provided withsealing means such as a succession of leakproof curtains positionedabove the glass ribbon. This minimizes the escape of the protectivereducing atmosphere present over the metal bath of the flotationfurnace. It is preferable to provide three curtains with their loweredges terminating immediately above the glass ribbon.

A more complete understanding of the invention can be obtained byreference to FIGS. 1 through 4 in conjunction with the followingdiscussion.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described hereinbelow withreference to the drawings wherein:

FIG. 1 is a side elevation in section of the glass ribbon extractionapparatus according to the present invention;

FIG. 2 is a sectional view taken along line A--A of FIG. 1;

FIG. 3 is a sectional view taken along line B--B of FIG. 1; and

FIG. 4 is a perspective view of an alternate embodiment of the coolinggrid of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates the downstream end of a flotation furnace. A tankcomprised of fire-resistant materials 4 is positioned within theflotation furnace 1. The exterior of the tank is protected by asheet-metal lining 5. The tank is filled with a molten metal bath 2which usually consists of molten tin. The flotation furnace 1 alsoincludes a roof 6, side walls 7 and an exit wall 8. An opening 14 isprovided in exit wall 8 for the passage of a glass ribbon 13. A metalplate 10 lines the lower portion 9 of the exit wall 8 and supports acooling grid 11. A protective reducing atmosphere, consisting of amixture of hydrogen and nitrogen, is maintained in the space 3 above thebath 2. A succession of gas-tight curtains 17a, 17b and 17c bars theexit opening 14 above the glass ribbon flow. Advantageously there arethree curtains to prevent efficaciously the ambient atmosphere fromcontaminating the protective atmosphere.

An enclosure in the form of a case 20 is positioned between thedownstream exit wall 8 of the flotation furnace and the inlet to aflattening fixture 16. Three adjustable extraction rollers 12a, 12b and12c are positioned in case 20 with their axis of rotation transverse tothe flow of the glass ribbon 13. The case 20 at least partially supportsthe rollers 12a, 12b and 12c. Glass ribbon 13 generally divides case 20into two portions with a U-shaped cradle 201 having side walls 26comprising the lower portion of the case 20 and a similarly U-shapedroof 202 comprising the upper portion of the case 20. The upstreamportion of roof 202 is removed to form a chimney 23. Alternatively, thecase 20 maybe of a unitary construction with its upstream upper portionremoved to form chimney 23.

The lower portion of the case 20 has its upstream and downstream ends(facing the exit of the furnace and facing the flattening fixture 16)sealed by transverse walls 28. The upper edge of each wall 28 reaches alevel below that of the upper generatrix of the extraction rollers topermit the glass ribbon 13 to pass thereabove. Alternatively, one orboth of the walls 28 may be removed.

The roof 202 rests above the downstream portion of the cradle 201 andencloses the last extraction rollers 12b and 12c. The first at leastextraction roller 12a is not covered by roof 202. It is topped bychimney 23 which is positioned between the exit wall 8 of the flotationfurnace and the upstream end of roof 202. The superposition of roof 202upon cradle 201 forms a thermal enclosure 24. Transverse gas-tightcurtains 22 and 25 are advantageously positioned at the entrance andexit of the roof 202 above the glass ribbon 13. This insulates theatmosphere within the thermal enclosure 24.

As shown in FIG. 1, cradle 201 is a carriage 27 mounted on rails to movetransversely to the flow of the glass ribbon. The roof 202 is maintainedin a working position by auxiliary means not shown in FIG. 1. Themobility of the cradle 201 provides easy access to the extension rollers12a, 12b and 12c. This accessibility can alternatively be obtained byconstructing the roof 202 so that it moves in a direction parallel tothe flow of the glass ribbon 13.

The arrangement of the components described above, produces verticalchannels 29 located between each of the two side walls 26 of cradle 201and glass ribbon 13. These vertical channels 29 are readily seen inFIGS. 2 and 3.

The cooling grid 11 advantageously comprises several tubes independentlysupplied with cooling fluid. Due to its position between plate 10 offlotation furnace exit wall 8 and the first extraction roller 12a, thecooling grid 11 might abnormally cool the roller 12a. For this reason, athermal screen or baffle plate 19 is positioned between the cooling grid11 and the first extraction roller 12a. The thermal screen 19 may be aninsulating plate, or more advantageously, a mask which additionallycovers the upper portion of the cooling grid 11.

FIG. 2 illustrates the upstream face of the roof 202 positioned abovethe cradle 201. Channels 29 are indicated between glass ribbon 13 andside walls 26 of the cradle 201. For clarity, the flattening fixture 16is not shown.

FIG. 3 is an upstream view of the flotation furnace 1 and the glassribbon extraction apparatus. The last gas-tight curtain 17c sealsopening 14 with the curtain stopping advantageously near yet notcontacting the glass ribbon 13. The cooling grid 11, shown behind thefirst extraction roller 12a, comprises an assembly of cold pinsindepently supplied with cooling fluid. The configuration of the pinspermits the inflow and outflow of fluid to occur on the same side of thecooling grid 11. The cooling fluid may be for example air, oil or water.For clarity, the thermal screen 19 and the wall 28 of cradle 201 havenot been shown.

FIG. 4 illustrates the downstream end of the tank of the flotationfurnace 1 including the metal bath 2, lower exit wall 9, metal plate 10and cooling grid 11. This grid 11 advantageously comprises a series ofhorizontal tubes 30 inserted in openings 31 and a series of flat ironvertical supports 32 attached onto the external face of metal plate 10by known means such as welding.

By separately adjusting the fluid flow in each of the tubes 30, thetemperature of various portions of metal plate 10 and lower exit wall 9may advantageously be regulated. Efficient cooling of this portion ofthe flotation furnace 1 is desired to protect the metal plate 10 fromattack by the molten tin contained in metal bath 2. The tin might haveseeped between the interstices of the fire-resistant materials 4 to themetal plate 10. Excess cooling near the outlet opening 14, however,should be avoided. Substantial cooling in this area gives rise toconvection currents in the metal bath 2. These currents carry tin oxidesto the exit wall 8 of the flotation furnace approxiamately at thelocation of the glass ribbon exit from the metal bath 2. These oxidescan contaminate the glass.

According to FIG. 4, the cooling fluid is supplied to each tube 30 atthe same side of the cooling grid 11 and is evacuated at the other sideof the grid 11. The tubes 30 may be lined with fins and can be locatedon two rows parallel to metal plate 10.

In the flotation furnace, the reducing atomsphere above the molten metalbath 2 consists preferably of a nitrogen-hydrogen mixture. Thisatomsphere is at a pressure greater than the atomspheric pressureoutside the furnace causing hot gases to escape form the furnace throughthe exit opening 14 in two streams. A first stream forms between theupper surface of the glass ribbon 13 and the leak-proof curtains 17a,17b and 17c. The first stream is generally small because the curtains17a,17b and 17c advantageously terminate at a short distance above theupper surface of the glass ribbon 13. A second, generally larger stream18 forms between the underside of the glass ribbon 13 and a lower lip 15of the exit opening 14. The hydrogen ignites at the exit of theflotation furnace with combustion occurring above and especially belowthe glass ribbon 13 toward the upstream portion of cradle 201. Theflames and hot gases produced by the combustion of the gases passunderneath the extraction roller 12a, through verticle channels 29,which are located between glass ribbon 13 and side walls 26 of thecradle 201, and exhaust through chimney 23 to the ambient atomsphere. Atypical flow of the flames and combustion gases is indicated in FIGS.1-3 by stippled arrows. The combustion of the gases creates a thermalbarrier between the surrounding atomsphere on either side of theflotation furnace line and the glass ribbon 13. This effect, combinedwith the thermal regulating effect of the thermal enclosure 24, protectsthe high temperature glass ribbon 13 from sudden or uneven cooling withits resulting thermal stresses. Such stresses are harmful to the qualityof the glass.

Thus, the present invention provides for the thermal regulation of theglass ribbon by preventing any sudden fall in its temperature. The glassribbon passing through the extracting apparatus is also retained at ahigh temperature by the extracting apparatus. It is possible, therefore,to perform various processes on the high temperature glass ribbon. Forexample, the glass ribbon could be coated with a metal oxide depositaccording to the methods described in French patent applicationspublished under numbers 2,211,411 and 2,277,049. The metal oxidedeposits are generally applied to the glass at a temperature at least ashigh as the temperature of the glass ribbon exiting the metal bath,i.e., at greater than 500° C. The short lock-chamber device according tothe invention makes it possible to apply this metal oxide depositwithout reheating the glass ribbon.

We claim:
 1. An apparatus for extracting a glass ribbon from an exit ofa flotation furnace sized to permit free passage of said glass ribbonfrom said furnace interior, which apparatus comprises:(a) a plurality ofextraction rollers arranged in succession adjacent the end portion ofthe flotation furnace for extracting the glass ribbon from said exit,said rollers disposed such that they elevate to create a gap beneathsaid glass ribbon at said exit whereby gases which combust as they passthrough said gap surround said glass ribbon and form a thermal barrier;and (b) enclosure means arranged adjacent the exit of the flotationfurnace for enclosing said rollers, said enclosure means having anopening for receipt of the glass ribbon exiting the flotation furnaceand having an opening for passage of the glass ribbon therefrom, saidenclosure means also having an opening located above at least the firstone of said rollers and adjacent the exit of the flotation furnace forventing said combustion gases to the ambient atomsphere.
 2. An apparatusaccording to claim 1 wherein the enclosure means comprises a case. 3.The apparatus according to claim 2 wherein the case comprises a U-shapedcradle and an inverted U-shaped roof positioned above and supported bythe cradle, said roof having an end nearer the furnace which ispositioned downstream of said first of said rollers and wherein saidopening for venting said combustion gases is defined by the spacebetween the exit end of the said furnace and the end of said roof. 4.The apparatus according to claim 3 wherein the cradle has side wallspositioned parallel to the glass ribbon flow with the extraction rollersrotatably mounted within the cradle between said side walls so as toposition the rotational axes of the extraction rollers transverse to thedirection of the glass ribbon flow, said side walls having upper edgeswhich terminate at least at a level equal to the level of the uppergeneratrix of the extraction rollers.
 5. The apparatus according toclaim 4 wherein the U-shaped cradle further comprises at least onetraverse wall connected to an end of the U-shaped cradle, the traversewall being positioned so that its upper edge terminates approximatelybelow the upper generatrix of the extraction rollers thus permitting theglass ribbon to pass thereabove.
 6. The apparatus according to claim 3further comprising means for sealing at least one of the end portions ofthe roof.
 7. The apparatus according to claim 6 wherein said roofsealing means comprises at least one gas-tight curtain positioned at theend portion of the roof above the glass ribbon.
 8. The apparatus ofclaim 7 wherein each gas-tight curtain of the roof sealing means extendsto a position closely adjacent said glass ribbon.
 9. The apparatusaccording to claim 3 wherein the cradle comprises a carriage havingmeans for moving the carriage transverse to the flow of the glassribbon.
 10. The apparatus according to claim 3 wherein the end of theroof is located to a position between a first and second of saidplurality of extraction rollers.
 11. The apparatus according to claim 1further comprising means for limiting the size of the exit above theglass ribbon for said free passage from said flotation furnace tothereby minimize the escape of the atmosphere within the flotationfurnace above the glass ribbon.
 12. The apparatus according to claim 11wherein said flotation furnace limiting means comprises a plurality ofgas-tight curtains positioned above the glass ribbon at the egress ofthe flotation furnace with the lower edge of at least one of thecurtains terminating immediately above said glass ribbon.
 13. Theapparatus according to claim 12 further comprising means for regulatingthe temperature of the egress of the flotation furance.
 14. Theapparatus according to claim 13 wherein said temperature regulatingmeans comprises a cooling grid secured to the flotation furnace withinthe region of the egrees below the glass ribbon.
 15. The apparatusaccording to claim 14 further comprising a thermal screen positionedbetween the cooling grid and the extraction roller adjacent saidflotation furnace so that said adjacent extraction roller is at leastpartially insulated from the cooling grid.
 16. The apparatus accordingto claim 15 wherein the cooling grid comprises a plurality of pipes,each pipe independently supplied with a cooling fluid for cooling theegress of the flotation furnace below the glass ribbon.
 17. Theapparatus according to claim 1 wherein the extraction rollers furtherare disposed within said enclosure means whereby said combustion gasespass underneath at least the extraction roller adjacent said flotationfurnace and said thermal barrier between said enclosure means and saidflotation furnace is maintained.
 18. Apparatus for use in combinationwith a flotation furnace for extracting continuously a ribbon of glassthrough an outlet and supporting the ribbon of glass in movement to adownstream processor wherein said apparatus comprises:(a) a plurality ofextraction rollers arranged in succession adjacent to the outlet of theflotation furnace; and (b) enclosure means arranged closely adjacent theoutlet of the flotation furnace and including(1) a lower portion havinga pair of side walls with an upper supporting surface and means definingboth an entrance to and an exit from the enclosure means, (2) an upperportion of an outline similar to the outline of the lower portionreceived on and supported by the supporting surface of the side walls,the upper portion being coextensive with the lower portion onlythroughout a part of its length thereby to provide a chimney of a sizeto include the area at least above the first of the extraction rollersof the plurality; and (c) the plurality of extraction rollers beingdisposed within the lower portion of the enclosure means and supportedby the side walls so that the upper generatrix of the extraction rollersis above the level of the entrance to and exit from the enclosure means,at a level no higher than the level of the supporting surfaces to permitpassage of the glass ribbon through said enclosure means and at a levelto elevate said glass ribbon at said outlet to create a gap therebeneathwhereby gases which combust as they pass through said gap surround saidglass ribbon and form a thermal barrier, said combustion gases exitingsaid apparatus through said chimney.
 19. The apparatus according toclaim 16 wherein the pipes of the cooling grid are cooling pins.